1. Field of the Invention
The present invention relates to a method of making flexible printed circuits for use in radio sets or like electronic appliances.
2. Prior Art
Hitherto, flexible printed circuits have been subjects of undesirable shrinkage due to internal stress residing in the polymer resin base film. Elucidating in detail, conventional flexible printed circuits are made by carrying out the following sequence of steps on a substrate consisting of an insulating polymer resin film, such as polyimide film and copper foils bonded on both surfaces:
punching through-holes and positioning holes,
electroless plating followed by electroplating of copper on the inner surface of the through-holes,
laminating a photopolymer film on the substrate,
exposing the photopolymer film to a photochemical light of a predetermined pattern,
developing the photopolymer film,
etching the copper film where not covered by the photopolymer film,
removing the photopolymer film, and
screen-printing of a solder-resist film.
In the abovementioned prior art method, the steps of the forming of through-holes and positioning holes, the forming of the photoresist lamination and the forming of the solder-resist film must be made with highly accurate registrations of the corresponding position on the substrate. In order to attain such accuracy or positionings, the conventional method of making the flexible printed circuits employs, instead of working on a continuous long belt of the substrate, the technique of cutting a long belt of substrate into pieces of a predetermined length of, for example 50 to 80 cm and carries out the subsequent steps thereon. Such a way of working on determinate-length pieces of substrate is not advantageous in the viewpoint of efficiency. However, handling of a long, continuous substrate with accurate registration of the performance of the several steps thereon has been difficult because of the shrinkage in the substrate due to internal stress of the polyimide film after removal of a considerable part of the copper foils thereon to form necessary wiring pattern. If the registration is not good, many adverse problems arise such that rounds are not formed around the through-holes, a solder-resist film is not rightly printed into the through-holes thereby causing undesirable filling of solder therein, leading to insufficient mechanical and insufficient electric connections of lead wires of electronic components into the through-holes or impossibility of insertion of the lead wires into the through-holes.